The present invention relates to an improvement in quick-setting magnesium phosphate-based cements. More particularly, this invention concerns an improvement of normally quick-setting cement and concrete compositions based upon magnesium oxide and ammonium phosphate, whereby the setting time of the composition is retarded to a desired degree by the incorporation of certain additives in the compositions. Additionally, an increase in the extended or overall compressive strength of the retarded compositions has been observed.
Plastic compositions having a very quick-setting time and prepared by mixing a magnesium oxide component with an ammonium phosphate component and usually an aggregate have become popular for use in repairing or patching cracks, holes and spalled areas of highways, airfield pavements and bridge decks, as well as for a variety of commercial and industrial applications. Compositions in particular described in U.S. Pat. No. 3,285,758 to Limes and Ponzani wherein a solid aggregate component containing magnesium oxide is mixed with an aqueous solution of ammonium polyphosphates have found successful application in these areas. The compositions set in substantially less than one half hour at 70.degree.F after mixing, and the strength development of these compositions is sufficiently rapid to permit for example automobile traffic thereover within a few hours following application. Additionally, the compositions exhibit excellent adhesion to Portland cement concrete, steel and to themselves, and attain high ultimate compressive strength and are durable.
It was found however that the normally quick-setting time of such compositions became even more rapid with increasing ambient temperature. Thus the work life of the composition upon mixing of the two reactive components became substantially reduced at the higher ambient temperatures. This reduction of the work life can pose a problem for example in cases where the compositions are utilized as patching or grouting materials for roadways during hot summer days when ambient temperatures rise significantly above 70.degree.F. The addition of greater proportions of water than normally called for to the compositions accelerated the setting time and adversely affected the set strength of composition. There is a need therefore for a practical method of retarding the setting time and thereby extending the work life of the composition without adversely affecting other desirable properties of the composition.